Molds, dies, cores, pins and other such forming members for shaping molten metals are frequently manufactured from steel because of its low cost and ease of fabrication. Problems arise when such forming members are used to shape reactive metals, such as aluminum, magnesium and zinc containing alloys. It has been found that such alloys, particularly low iron content aluminum alloys, are reactive in the molten state and can corrode and/or erode the surface of molds, dies and other forming members significantly reducing their surface life.
A number of approaches have been taken in an attempt to solve this problem. One approach involves nitriding the surface of the forming member. Nitrided surfaces do provide some protection from attack by certain alloys; but this protection is fairly limited, and nitrided surfaces are ineffective against more reactive metals, such as aluminum alloys, particularly those aluminum alloys having an iron content of 1.2% or less, and aluminum alloys with a silicon content of up to 18%. Another approach involves implanting tungsten into the surface of steel forming members. Generally, such implantation processes are fairly expensive; furthermore, protection provided thereby is still somewhat limited.
A further approach, which is disclosed in U.S. Pat. 5,476,134, comprises coating a surface of the forming member with a layer of chromium nitride. As disclosed therein, the chromium nitride coating has good resistance to attack by reactive metal alloys, including low iron content aluminum alloys. While such prior art chromium nitride coatings provide good erosion resistance, it has been found that these coatings tend to fail, particularly in the region of sharp features on the forming member such as edges, textured surfaces and the like. While not wishing to be bound by speculation, the inventors herein theorize that stresses in such prior art coatings concentrate at sharp edges and provide cracks and fissures which allow molten metal to contact underlying substrate material and cause erosion.
The foregoing makes clear that there is a need for a surface treatment which can be applied to forming members such as molds and dies, which will protect such members from attack by reactive metals. The treatment should be easy to apply, low in cost, and should not interfere with use of the forming members. In addition, the treatment should be durable and provide long service life. The present invention, as will be described in detail hereinbelow, utilizes a doped coating of chromium nitride as a protective treatment for forming members. While chromium nitride coatings have previously been used in molds for shaping aluminum, doped coatings have not; and it has been found, unexpectedly, that the addition of relatively small amounts of dopant greatly enhance the resistance of chromium nitride coatings to attack by molten metal. These and other advantages of the present invention will be apparent from the drawings, discussion and description which follow.